1. Field of the Invention
The present invention relates to a print head drive unit used in an ink jet or other type of printer.
2. Description of the Related Art
FIG. 1 shows a conventional ink jet head 100 used in an ink jet printer to eject ink droplets. The ink jet head 100 includes a chamber block 103 and a piezoelectric element 122. The chamber block 103 is formed with a pressure chamber 116, a manifold 124, and an ejection nozzle 120. The pressure chamber 116 is filled with ink. The piezoelectric element 122 is fixed on the upper wall of the chamber block 103 and is connected to a drive circuit 110. To eject ink droplets 126 from the ejection nozzle 120, the drive circuit 110 applies a voltage pulse to the piezoelectric element 122 so that the piezoelectric element 122 deforms. The upper wall of the chamber block 103 deforms accordingly as indicated by dotted line in FIG. 1. When the upper wall of the chamber block 103 deforms into the pressure chamber 116 in this manner, the pressure in the pressure chamber 116 increases and pushes ink out from the pressure chamber 116 and the nozzle 120 in the form of ink droplets 126.
As shown in FIG. 2, an actual ink ejection head 101 includes a plurality of pressure chambers 116 and nozzles 120. Piezoelectric elements 122 are provided on confronting walls that form the pressure chambers 116. The pressure chambers 116 and the nozzles 120 are aligned in an auxiliary scan direction in which recording sheets are transported past the ink ejection head 101. Printing is performed by applying drive voltage pulses selectively to the piezoelectric elements 122 while the print head 101 is being transported in a main scan direction, which is perpendicular to the auxiliary scan direction of sheet transport.
In order to increase print speed, some printers use print heads 101 with an increased number of ejection nozzles 120. Some printers use more than one print head 101 aligned in an array. In order to improve quality of printed images, some printers use a greater number of print heads 101 to enable printing using different colored inks.
Because conventional ink jet printers can have such a large number of ejection nozzles 120 and heads, the chance that the piezoelectric elements 122 of different ejection nozzles 120 will be applied with drive voltage simultaneously is quite high. If drive voltage is applied simultaneously to different piezoelectric elements 122 in this way, the flow of drive current to the different piezoelectric elements 122 will peak at the same time, so that drive voltage drops. The drop in voltage degrades ejection characteristics, such as speed at which the ink droplets 126 are ejected from the nozzles 120, resulting in inferior image quality.
To prevent such a drop in drive voltage, Japanese Patent Application Publication Nos. 9-262974, 9-262978, and 9-272200 disclose shifting current peaks beforehand by a predetermined duration of time in an attempt to prevent current peaks from overlapping.
However, this conventional method is insufficient for situations when a great variety of different and complicated waveforms are used. For example, recently ink-jet printers have been developed that are capable of gradation printing, that is, capable of printing in a variety of different tones. Such printers use a variety of different waveforms. Each waveform includes a plurality of drive voltage pulses, and each pulse includes a rising edge and a lowering edge. The plural drive voltage pulses in the waveforms are for ejecting a plurality of ink droplets at the same time or canceling out residual pressure waves after ink ejection. When the waveforms are merely shifted by a predetermined duration of time as in the conventional method, there may be times when the current peaks overlap because of the large number of, and complicated nature of, the waveforms.
To overcome this problem, it is conceivable to modify the shape of the drive waveforms themselves so that the rising and lowering edges of the drive waveforms do not overlap. However, this would influence the size of ejected ink droplets and optimum printing speed so that quality printing cannot be achieved.
It is an objective of the present invention to overcome the above-described problems and to provide a drive unit that is capable of reliably preventing overlap in high current times of different heads or different sections of the same head.
In order to achieve the above-described objectives, a drive unit according to one aspect of the present invention is for driving a print head unit including a plurality of actuators, wherein the drive unit includes a drive circuit, a memory, and a drive circuit control unit. The drive circuit selectively applies drive waveforms of a plurality of drive waveforms to the actuators of the print head unit to drive the actuators. The memory is prestored with a high current time for each of the plurality of drive waveforms. Each high current time represents a time of high current flow resulting from the drive circuit applying the corresponding drive waveform to the actuators. Based on the high current times stored in the memory, the drive circuit control unit controls the drive circuit to apply drive waveforms to different sections of the print head unit at timings with no overlap in high current times of the drive waveforms applied to the different sections.
According to another aspect of the present invention, a drive unit is used for independently driving at least two different sections of a print head unit and includes a memory, a print timing judge unit, a comparator, and a print operation delay unit. The memory stores timing maps that indicate rising edges of drive waveforms used to drive the print head unit. The print timing judge unit judges then one of the sections of the print head unit is to be driven to perform a print operation. If the print timing judge unit judges that the one section is to be driven, the comparator compares the timing maps in the memory to find rising edges that overlap between a timing map that corresponds to a drive waveform used to drive the one section and a timing map that corresponds to a drive waveform used to drive another section of the print head unit. When the comparator finds rising edges that overlap, the print operation delay unit delays drive of the one section until the comparator no longer finds rising edges that overlap after the comparator shifts, according to the delay, the timing map that corresponds to the drive waveform used to drive the one section.
A method according to the present invention is for independently driving at least two different sections of a print head unit. The method includes the steps of judging when one of the sections of the print head unit is to be driven to perform a print operation; comparing, when the one section is to be driven, timing maps that indicate rising edges of drive waveforms used for driving the print head unit; and delaying, when rising edges are found to overlap between a timing map that corresponds to a drive waveform used to drive the one section and a timing map that corresponds to a drive waveform used to drive another section of the print head unit, drive of the one section while shifting, according to the delay, the timing map that corresponds to the drive waveform used to drive the one section until no rising edges are found to overlap.